There is an ongoing need for acoustic wave filters and delay lines for a variety of practical applications. Acoustic wave devices are becoming particularly important in the production of electronic signal processing equipment, especially radios, because they can be readily mass produced and are of small size and also because of increased pressure to employ available radio spectrum efficiently. Acoustic wave devices are generally constructed on planar surfaces using techniques similar to those employed in manufacturing integrated circuits.
A number of low-loss acoustic wave filter and transducer design approaches including temperature compensation, broad bandwidth and improved out-of-band signal rejection have been developed to meet specific performance goals relevant to particular applications and subject to specified manufacturing constraints. These approaches include multiphase uni-directional acoustic wave transducers, multi-transducer acoustic wave filters, resonators, distributed acoustic reflection transducers (DARTs) and acoustic wave transducers backed by reflectors, with each acoustic device type providing filter performance strengths and weaknesses. Specific substrate orientations and materials have also been developed to provide improved acoustic wave propagation and transduction characteristics, including increased electromechanical coupling coefficients, decreased temperature sensitivity, decreased acoustic propagation loss, decreased design and fabrication sensitivity, as well as reduced complexity and other factors.
Examples of these and other types of acoustic wave device approaches including choices of substrate materials and orientations are described in "propagation and Amplification of Rayleigh Waves and Piezoelectric Leaky Surface Waves in LiNbO.sub.3 " by K. Yamanouchi and K. Shibayama, J. App. Phys. Vol. 43, No. 3; U.S. Pat. Nos. 4,159,435, entitled "Acoustic Surface Wave Devices Employing Surface Skimming Bulk Waves", by M. F. Lewis; 4,489,250, entitled "Temperature Compensated Surface Acoustic Wave Device" by Y. Ebata et al.; 4,670,680, entitled"Doubly Rotated Orientations of Cut Angles For Quartz Crystal For Novel Surface Acoustic Wave Devices" by J. Andle; 4,525,643 and 4,511,817, entitled "Temperature Compensated Orientations of Berlinite for Surface Acoustic Wave Devices" by B. Chai et al.; and 4,001,767, entitled "Low Diffraction Loss-Low Spurious Response LiTaO.sub.3 Substrate for Surface Acoustic Wave Devices" by A. Slobodnik, which are hereby incorporated herein by reference. However, these and other prior art approaches suffer from a number of disadvantages well known in the art. These disadvantages tend to become more serious as operating frequency increases.
Of particular interest are substrata supporting surface skimming bulk acoustic waves. These tend to provide increased electromechanical coupling coefficients, resulting in a desirable combination of insertion loss, out-of-band rejection, bandwidth and other characteristics. Moreover, the reduced sensitivity of device insertion loss to surface contamination as a result of the horizontally polarized particle motion associated with such waves potentially greatly simplifies device packaging and manufacturing constraints, encouraging reduced manufacturing costs, package footprint and weight and these trends are also consistent with increased robustness of acoustic wave devices including such substrata. As noted in FIG. 3 and associated text of Yamanouch (supra), losses due to scattering into bulk acoustic waves for Y-cut, rotated LiNbO.sub.3 are a strong function of cut angle. Minima in these losses occur at different rotation angles depending on whether the substrate surface is open- or short-circuited. 41.degree. and 64.degree. rotated, y-cut LiNbO.sub.3 substrata have minimum insertion losses for the open- and short-circuited cases, respectively. Because neither condition corresponds to that obtaining in an interdigitated transducer, some scattering of acoustic energy into other, undesired acoustic propagation modes, and hence some additional component or filter insertion loss, is noted in devices employing such transducers.
What are needed are substrata and frequency selection components, as well as methods for providing and using same, having the advantages of broad bandwidth and generally reduced insertion losses associated with surface skimming bulk waves (compared to other technologies) but with reduced insertion losses, especially those losses associated with acoustic propagation loss occurring in partially metallized regions such as acoustic wave transducers.